Swift prospects for gamma-ray bursts

Physics World, October 2004

This October* sees the launch of the first observatory dedicated to finding and studying gamma ray bursts as they happen. Gamma ray bursts (GRBs) are one of the most spectacular but least understood of astrophysical phenomena. What is known is that each one releases more energy in a few seconds than our sun will release in its lifetime, and that they occur early in the history of the universe.

The Swift observatory is the third mission of Nasa's medium-class explorer (Midex) program, following the Image magnetosphere probe and the Wilkinson Microwave Anisotropy Probe (WMAP). The $160m mission is headed by Goddard Space Flight Center, Maryland, and Penn State University, with partners in Italy and the UK.

Swift's main instrument is a gamma ray telescope which covers a sixth of the sky. Once this sees the signs of a GRB, the craft reorients itself to bring an optical and x-ray telescope to bear within one minute.

"There's no other burst mission that has this complement of different telescopes," says Neil Gehrels, principal investigator for the mission at Goddard. "The capability to repoint itself and the speed with which it does it are really unique capabilities. The most important thing you need for studying bursts is to look at the afterglow immediately afterwards. It decays away quickly, so time is of the essence."

Bursting with questions
During the initial two-year mission, Swift is expected to observe at least 200 bursts and compile the most comprehensive study of GRBs and their afterglows to date. Because gamma rays are absorbed by the Earth's atmosphere, you have to go into space to watch for bursts.

GRBs were first spotted in 1967 by US military satellites looking for evidence of nuclear testing – although thanks to the secret nature of the satellites, the news wasn't reported until 1973. Because GRBs appear so bright, astronomers thought they had to be within our own galaxy. That changed in 1997, when the Italian-Dutch x-ray satellite BeppoSax discovered that GRBs had an afterglow, which rapidly decays down the electromagnetic spectrum through the x-ray to the optical band within minutes of the burst. The spectra of these afterglows showed that the bursts occurred at high redshifts – far outside our own galactic cluster, and early in the lifetime of the universe.

Astronomers now think that GRBs are associated with the formation of black holes. "The current idea is that a massive star reaches the end of its life and explodes, not as a supernova but as a hypernova," says Gehrels. "Instead of the core collapsing to a neutron star, it collapses to a black hole."

If GRBs are caused by collapsing stars, the most distant bursts are likely to come from the first massive stars to be formed in the early universe. "Because gamma ray bursts are so penetrating we can look back to redshifts of 15-20 – that's the only way we can study this first generation of stars," Gehrels notes.

The make-up of the interstellar and intergalactic media can also be studied by observing the afterglow, thought to be produced by the burst's shockwave travelling at a significant fraction of the speed of light.

The mission also aims to compile a taxonomy of GRBs and to study their distribution. There's currently thought to be two main types of GRB with quite different gamma-ray profiles – ones that last longer than two seconds, which are thought to be caused by hypernovae; and shorter bursts that can last only a few milliseconds. The shorter ones are particularly mysterious as no afterglows have been observed.

The puzzle deepened recently with the confirmation of a new class of GRB which is considerably weaker than others but much more powerful than supernovae, suggesting a continuum between the two phenomena. In December 2003, the European-Russian Integral satellite observed a burst labelled GRB031203. At a distance of 1.6 billion light years (redshift of 0.105), the burst was the closest to Earth ever studied but released only a thousandth of the typical gamma ray energy.

"That was a really exciting discovery from Integral," Gehrels notes. "Swift is perfectly designed for studying that, because it's much more sensitive than any gamma ray detector that's flown. Even though they're nearby, they're still quite faint."

New era of technology
The Swift craft brings a mixture of new and proven technologies to its task. Its primary instrument, the Burst Alert Telescope (BAT), uses an array of over 30,000 tiny cadmium-zinc-telluride (CZT) detectors. Similar solid-state detectors were pioneered by Integral, which used cadmium telluride detectors.

"These detectors are really ushering in a whole new era in gamma ray technology," says Gehrels. "They're denser and have a higher atomic number, so you can use smaller detectors and use whole arrays of them."

Swift is also unique in the speed with which it can it can reorient itself to aim its secondary telescopes to capture the immediate afterglow of a burst. In around a third of cases, the x-ray and optical/ultraviolet telescopes should be trained on the GRB while the burst itself is still happening. This is achieved by using six momentum wheels to turn the craft rather than the three or four used on other satellites.

But the biggest challenge was programming the craft's computers. "They have to do all of the constraint checking, to make sure the spacecraft isn't going to be pointed at the sun or bright moon or the earth, which could damage the instruments," says Gehrels.

The x-ray and optical telescopes are adapted from those used on previous missions, including ESA's XMM-Newton x-ray observatory. These will pinpoint the location of the burst to within four arc minutes (equivalent to one eighth the apparent size of the moon). The location and initial data will then be transmitted to a network of ground-based and orbital observatories to allow them to study the afterglow.

Although Swift is focusing on GRBs in its first two years of operations, it can do much more. Even in its first year, the BAT telescope will be carrying out the first hard x-ray survey of the sky for 25 years.

If funding is extended, Swift could study other transient phenomena in the gamma ray sky, from active galactic nuclei to such exotica as pulsar glitches and dwarf novae outbursts. "There's literally hundreds of these transient sources and many of them are poorly understood," Gehrels says. "The Swift observatory is perfectly designed for that."

* - following the usual delays, Swift launched on 22 November 2004